1. Field of the Invention
The present invention relates to a multi-screen projector having at least two unit module projectors stacked one upon another, and more particularly to a means for converging output light from unit module projectors to audiences.
2. Description of the Related Art
A conventional multi-screen projector having at least two unit module projectors stacked one upon another has a structure such as shown in FIG. 1. A higher projector unit 1a and a lower projector unit 1b respectively for higher and lower unit module projectors each have an image receiving tube and a projection lens. A higher screen 2a and a lower screen 2b are used respectively for the higher and lower unit module projectors.
In FIG. 1, the two unit module projectors piled up one upon the other constitute one multi-screen projector. The higher projector unit 1a and higher screen 2a constitute the higher unit module projector, and the lower projector unit 1b and lower screen 2b constitute the lower unit module projector. Each unit module projector has a screen size of 70 inches. The front and back rows of audiences are positioned remote from the screens 2a and 2b by 6 m (L6000) and 12 m (L6000+L3000+L3000), respectively. The center S of audiences is set at the middle point between the eyes of a standing audience and the eyes of a sitting audience at the middle row remote from the screens 2a and 2b by 9 m.
Each of the screen 2a and 2b has two sheet means, a Fresnel sheet means on the light input side and a lenticular sheet means on the light output side. Light of an enlarged image projected from the projector units 1a and 1b onto the screens 2a and 2b is converged to a predetermined position of audiences. The optical axis of the projection lens of the higher projector unit 1a coincides with the optical axis of the higher screen 2a, and the optical axis of the projection lens of the lower projector unit 1b coincides with the optical axis of the lower screen 2b, the axes being in parallel with the plane on which the multi-screen projector is installed. The optical axis of the lower screen 2b passes through the center S of audiences. The Fresnel sheet means and lenticular sheet means of the screens 2a and 2b are so structured that the convergence points (Fresnel conjugate points) k.sub.2 and k.sub.1 are on the optical axes of the screens 2a and 2b near a line perpendicular to the installation plane and passing through the center S of audiences. The Fresnel conjugate point k.sub.1 of the lower screen 2b coincides with the center S of audiences, and the Fresnel conjugate point k.sub.2 of the higher screen 2a is at the position higher than the Fresnel conjugate point k.sub.1 by the height of the screens 2a and 2b.
With the configuration of the multi-screen projector described above, the line A of sight of a sitting audience at the front row extending to the interface b between the screens 2a and 2b is deviated by about 10.degree. from the main output light from the lower end b of the higher screen 2a, and is slightly deviated by about 3.degree. from the main output light from the upper end b of the lower screen 2b. Light diffused from the screens 2a and 2b becomes dark as it becomes remote from the main output light. Because there is a large difference of angles, an audience at the front row visually senses a large difference of brightness between the higher and lower screens 2a and 2b, loosing uniformity of brightness of the multi-screen. This phenomenon is also true for sitting audiences at the middle and back rows.
FIG. 2 shows another example of a conventional multi-screen projector. Like elements to those shown in FIG. 1 are represented by using identical reference numerals and characters.
In FIG. 2, a line B is in parallel with the installation plane and is extending to the interface b between the screens 2a and 2b. The optical axes of the screens 2a and 2b are symmetrical with the parallel line B, and the Fresnel conjugate point k of the screens 2a and 2b is an intersection between the parallel line B and a line perpendicular to the installation plane and passing through the center S of audiences.
The direction of the main output light from the lower end b of the higher screen 2a is the same as that of the main output light from the lower end b of the lower screen b. Therefore, relative to both the main output light beams, the angles of the line A of sight of a sitting audience at the front row extending to the interface b between the screens 2a and 2b are the same and take a value of about 6.degree.. The sitting audience at the front row visually senses a uniform brightness of the multi-screen 2 made of the screens 2a and 2b.
However, the main output light beams converge to the position higher than the center S of audiences. Therefore, the line A of sight of audiences becomes remote from the main output light beams, darkening the multi-screen 2 as a whole.
One of the methods of solving such a problem is disclosed, for example, in JP-U-63-56881 and JP-A-60-229586. According to this related art, as shown in FIGS. 3A, 3B, and 3C, of a Fresnel sheet means 3, a decenter of a Fresnel lens of a Fresnel sheet means (higher Fresnel sheet means) 3a for the screen 2a is set much lower at D160 (160 mm), whereas a decenter of a Fresnel lens of a Fresnel sheet means (lower Fresnel sheet means) 3b for the screen 2b is set slightly higher at D11 (11 mm). As a result, the main output light from the higher screen 2a is converged much lower, and the main output light from the higher screen 2b is converged slightly higher. In this manner, the Fresnel conjugate point k.sub.2 of the higher screen 2a shown in FIG. 1 is made coincident with the center S of audiences, and both the main output light beams from the screens 2a and 2b are converged to the center S of audiences. The "decenter" means a distance from a geometric center of a Fresnel sheet to an optical axis of the Fresnel lens.
Another method is disclosed, for example, in U.S. Pat. No. 4,919,518. As shown in FIGS. 4A, 4B, and 4C, a prism sheet means 5 is interposed between a Fresnel sheet means 3 and a lenticular sheet means 4 of each of the screens 2a and 2b. A prism angle .theta..sub.1 of the prism sheet means for the higher screen 2a is set larger than a prism angle .theta..sub.2 of the prism sheet means for the lower screen 2b. With the different prism angles, the main output light beams from the screens 2a and 2b are converged to the center S of audiences.
With the method illustrated in FIGS. 3A to 3C, it is necessary to set the decenter amount of the Fresnel lens of the higher Fresnel sheet means 3a to be much larger than the decenter amount of the Fresnel lens of the lower Fresnel sheet means 3b. For example, in the case of the unit module projector having a screen size of 70 inches shown in FIG. 1, a decenter amount D of the Fresnel lens of the Fresnel sheet means 3a required for the main output light from the higher screen 2a to be converged to the center S of audiences, is given by: EQU D=L * tan S=1740 * tan 5.3.apprxeq.160 mm,
assuming that a projection distance L between the projection lens and screen is 1740 mm, and an inclination angle of the line interconnecting the projection lens and the center S of audiences is 5.3.degree..
In order to have a decenter amount of 160 mm of the Fresnel lens, a Fresnel sheet means having a size of 80 to 90 inches is cut along three cut lines shown in FIG. 5. A manufacturing apparatus for a Fresnel sheet means having a size of 80 to 90 inches becomes bulky and raises the cost.
Furthermore, the prism angles of the Fresnel lens of the Fresnel sheet means 3a become large at the upper right and left corners of the higher screen 2a because of too a large decent amount D of the Fresnel lens. Therefore, aberration is generated in red, blue, and green output light, changing colors and posing a problem of a difficulty in practical use.
With the method illustrated in FIGS. 4A to 4C, because of different prism angles, the prism sheet means 5 of the higher and lower screens 2a and 2b are required to be manufactured by using different metal molds which are expensive, raising the cost of a multi-screen projector even it has only two unit module projectors.